In wireless communication systems, the recent trend is that a demand for high-speed data services continuously increases from the traffic perspective, and data services are provided mainly in a specific small area from the coverage perspective. Accordingly, developers and service providers of wireless communication systems have concerns regarding the small-size cells such as a pico cell and a hot zone.
Generally, a pico cell has the following characteristics. The pico cell has a smaller coverage than a macro cell and may overlap with the macro cell. Also, the pico cell operates in the same or a different frequency from the macro cell, and uses a low transmit power compared to a macro BS. In some situations, the pico cell can be accessed only by some licensed users or can be accessed by all users.
In recent years, the standards group, i.e. the Third Generation Partnership Project Radio Access Network Working Group 1 (3GPP RAN WG1), is considering heterogeneous networks. A heterogeneous network refers to a cellular deployment of a form in which Base Stations (BSs) using a low transmit power are overlapped with each other within the coverage of a macro BS. That is, cells of different sizes are mixed or overlapped with each other. However, BSs managing the respective overlapped cells make use of the same wireless technology. A BS using less transmit output in the heterogeneous network can be a micro BS, a Remote Radio Head (RRH), a pico enhanced Node B (eNB), a home eNB, a femto BS, a relay node and the like. For example, the heterogeneous network can be constructed according to FIG. 1.
FIG. 1 illustrates a heterogeneous wireless communication system according to the conventional art. Referring to FIG. 1, a plurality of pico cells 110-1 and 110-2, a plurality of femto cells 120-1 to 120-3, and a plurality of relay nodes 130-1 and 130-2 are overlapped within a macro cell 100, and each cell provides service to Mobile Stations (MSs) located within its own coverage. At this time, the MSs located in the coverage of the plurality of pico cells 110-1 and 110-2, the plurality of femto cells 120-1 to 120-3, and the plurality of relay nodes 130-1 and 130-2 are also located in the coverage of the macro cell 100; however, the MSs have preferential access to a micro cell with an excellent channel state rather than the macro cell 100. That is, if the MS accessing the macro cell 100 enters the coverage of the micro cell, the MS performs handover to the micro cell from the macro cell 100.
When an MS moves into a macro cell, a situation of FIG. 2 may occur. FIG. 2 illustrates an example of a movement path of an MS in a heterogeneous wireless communication system according to the conventional art. In FIG. 2, an MS 230 moves while connecting with a macro BS 200 within a cell of the macro BS 200. According to the movement of the MS 230, the MS 230 passes a cell edge portion of a pico BS 210. So, the MS 230 that has access to the macro BS 200 during a time (t1) 251 is located within a cell of the pico BS 210 during a time (t2) 252, and performs handover to the pico BS 210. Then, the MS 230 leaves the cell of the pico BS 210.
When leaving the cell of the pico BS 210, the MS 230 should again perform handover to the macro BS 200 to maintain seamless service. At this time, if the MS 230 is moving quickly, the time (t2) 252 when the MS 230 stays in the cell of the pico BS 210 would be very short. As such, it fails to ensure enough time to perform a handover, so a Radio Link Failure (RLF) may take place before the handover to the macro BS 200 is completed, causing data loss, service interruption and such. Particularly, in a real-time service such as Voice over Internet Protocol (VoIP), service interruption felt by a user may be critical. Furthermore, during handover from the macro BS 200 to the pico BS 210, the macro BS 200 discards context information of the MS 230. As such, attempts by the MS 230 to reconnect to the macro BS 200 will take a long time.
As described above, in an environment where a macro cell and a micro cell are overlapped, a situation for re-handover to the macro cell can occur. But, as an MS moves quickly, it is impossible to ensure enough time to perform handover. Furthermore, it is also difficult to reconnect to a macro BS. Thus, there is a need to propose an alternative for minimizing service interruption by quickly reconnecting to the macro BS in the above situation.